1. Field of the Invention
This invention relates to an optical transmission device, and more particularly to an optical transmission device using a dielectric thin film.
2. Description of the Prior Art
Since light is one kind of electro-magnetic energy which is the same as electric waves of the microwave band or the like, it is theoretically possible to construct an optical transmission device by diminishing a microwave transmission device in size. However, visible radiation of for example, red light has a wavelength of 0.6.mu., while microwaves of 5GHz have a wavelength of 6cm, and the ratio of their wavelengths is in the order of 10.sup.5 and this presents various problems.
To avoid these problems, there has heretofore been proposed a multilayer dielectric transmission device such as shown in FIG. 1. In FIG. 1, a dielectric thin film 2 of a refractive index n1 is formed on a dielectric substrate 1 of a refractive index n2 and a dielectric layer 3 of a refractive index n3 is formed on the dielectric thin film 2. Assuming that n2=n3=1.5 and that n1 - 2/n1 - n1 = 2.5 .times. 10.sup.-.sup.2, where a working wavelength is 1.mu., it is possible to transmit only the fundamental mode with the thickness d of the dielectric thin film 2 being about 0.6.mu..
It has also been proposed to use an optical transmission device such as depicted in FIG. 2, in which a dielectric thin film 11 having a width a, a thickness b and a refractive index n1 is surrounded by dielectrics 12 of refractive indexes n2, n3, n4 and n5. Further, an optical transmission device such as illustrated in FIG. 3 has also been proposed in which a dielectric thin film of a refractive index n1 is formed on a dielectric substrate 13 of a refractive index n4. However, in the cases of FIGS. 2 and 3, if the refractive index n1 is several percent greater than those n2, n3, n4 and n5, it is necessary to select the width a and the thickness b of the dielectric thin films 11 and 11a less than several microns. In this case, uneveness of the interface between the dielectric thin film 11 and other dielectrics (including air) presents a problem, and it is practically impossible to decrease the uneveness of the interface to such an extent as to be negligible with respect to a transmission wavelength.